Washington — How often is Washington, D.C., shaken by an actual earthquake? About as often as it is trembles with a political earthquake.

In other words, in the region of our nation’s capital, the ground shakes, glasses rattle, and people look quizzically around their homes every six years or so, on average. The area has been struck by tremors of magnitude 2.5 and above six times since 1974, according to US Geological Survey records.

Look back at the record of power switching from one party to another, in either Congress or the White House, and something that could earn the “earthquake” label tends to happen on about that same schedule.

But most of the actual earthquakes of recent decades weren’t as big as the DC earthquake that occurred Friday morning. It came in at a magnitude of 3.6 – making it the most severe such incident of the past 35 years.

The July 16 temblor was centered north of suburban Rockville, Md., about 35 miles northwest of D.C. proper. It was strong enough to wake sleepers and cause a flood of calls to 911. Reporters asked President Obama whether he felt the shock when the president appeared to discuss progress in the Gulf oil disaster.

Quakes may be rare in the Potomac region, but some have been memorable. In 1973, for instance, many residents of the mid-Atlantic were jolted from sleep by a temblor near the Pennsylvania-New Jersey-Delaware border. In 1969, West Virginia was hit by a 4.3 earthquake, and the shock waves could be felt as far away as D.C.

And while the West Coast may be the most quake-centric region in the country, eastern states have experienced some of the worst temblors. The great Charleston, S.C., earthquake of 1886 measured an estimated 7.3 and killed 60 people. It affected an area with a radius of 800 miles, including all parts of Maryland.

The largest eastern earthquake ever – and possibly the largest earthquake to strike North America since its settlement by Europeans – occurred in a series of three shocks near New Madrid, Mo., in 1811 and 1812. These temblors affected an area 10 times larger than that hit by the San Francisco earthquake of 1906, according to the US Geological Survey.